Fuel cells, fuel cell systems and interconnects for fuel cells and fuel cell systems remain an area of interest. Some existing systems have various shortcomings, drawbacks, and disadvantages relative to certain applications. Accordingly, there remains a need for further contributions in this area of technology.
A solid oxide fuel cell may be an electrochemical system configured to convert fuel (e.g., hydrogen) to electricity at relatively high temperatures (e.g., greater than about 500 degrees Celsius). In some examples, lower power degradation rate and lower cost can be achieved when operating these systems at lower temperatures. However, polarization of the cathode of the fuel cell may be relatively high at lower temperatures, which can affect system performance.
In some examples, cathodes may be formed of lanthanide nickelate having the general formula Ln2NiO4+δ. Lanthanide nickelates may have a layered structure with alternating layers of perovskites and sodium chloride type layers. The interstitial oxide-ions are accommodated by the mismatch of the equilibrium (Ln-O) and (M-O) bond lengths where the structural tolerance factor t is less than 1. This highly mobile O2− exhibits a good ionic conductivity. Moreover, in this structure, the Ni (III)/Ni (II) redox couples are pinned at the top of the O2−:2p6 bands to give an acceptably high electronic conductivity in the mixed-valence state. Due to its unique structure, lanthanide nickelate cathodes may have lower activation energy than other cathode materials being used for solid oxide fuel cells, such as LSM and LSCF. Further, lanthanide nickelate cathode polarization resistance may be less dependent on temperature change than other materials. Therefore, this material may maintain lower ASR at lower operating temperatures. Especially low ASR has been demonstrated from praseodymium nickelate cathode. However, one issue is that nickelate materials can be unstable under fuel cell operating temperatures, such as between about 700 to about 900 degrees Celsius. For example, under fuel cell operating conditions, the favorable phase of the nickelate cathode tends to decompose into undesired phases, which causes fuel cell performance degradation.
Due to its lower ASR, especially at lower temperatures, nickelate cathodes continue to be of interest in the field of fuel cells. In some examples, A-site doping, such as Sr or Ca, and B-site doping, such as Cu, Co, Fe, etc., may be employed in an attempt to stabilize nickelate phase. However, such attempts have achieved limited success and/or other issues were present, such as higher coefficient of thermal expansion (CTE) of the cathodes, resulting in a mismatch with other fuel cell materials or substrate.
Analysis has indicated that nickelate decomposition initiated from element exsolution from the A-site of a doped nickelate, such as Pr exsolution from Pr2NiO4, may result in the formation of oxide. When too much A site element exsolutes form nickelate, Ni may become rich on the B-site, and eventually exsolutes from B-site to form NiO. Analysis also indicates that exsoluted A-site element tends to diffuse into a cathode interlayer made from doped ceria on top of a stabilized zirconia electrolyte.